Continuous slurry polymerization volatile removal

ABSTRACT

A process/apparatus is disclosed for continuously separating a liquid medium comprising diluent and unreacted monomers from a polymerization effluent comprising diluent, unreacted monomers and polymer solids, comprising a continuous discharge of the polymerization effluent from a slurry reactor through a discharge valve and transfer conduit into a first intermediate pressure flash tank with a conical bottom defined by substantially straight sides inclined at an angle to that of horizontal equal to or greater than the angle of slide of the slurry/polymer solids and an exit seal chamber of such diameter (d) and length (l) as to maintain a desired volume of concentrated polymer solids/slurry in the exit seal chamber such as to form a pressure seal while continuously discharging a plug flow of concentrated polymer solids/slurry bottom product of said first flash tank from the exit seal chamber through a seal chamber exit reducer with inclined sides defined by substantially straight sides inclined at an angle to that of horizontal equal to or greater than the angle of slide of the polymer solids which remain after removal of about 50 to 100% of the inert diluent therefrom to a second flash tank at a lower pressure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of Ser. No. 09/654,799,filed Sep. 5, 2000, now allowed, which is a continuation application ofSer. No. 09/081, 392, filed May 18, 1998, now U.S. Pat. No. 6,281,300,which claims the benefit of and priority to Ser. No. 60/078,859, filedMar. 20, 1998, all of which are incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not applicable.

BACKGROUND OF THE INVENTION

In many polymerization processes for the production of polymer, apolymerization effluent is formed which is a slurry of particulatepolymer solids suspended in a liquid medium, ordinarily the reactiondiluent and unreacted monomers. A typical example of such processes isdisclosed in Hogan and Bank's U.S. Pat. No. 2,285,721, the disclosure ofwhich is incorporated herein by reference. While the polymerizationprocesses described in the Hogan document employs a catalyst comprisingchromium oxide and a support, the present invention is applicable to anyprocess producing an effluent comprising a slurry of particulate polymersolids suspended in a liquid medium comprising a diluent and unreactedmonomer. Such reaction processes include those which have come to beknown in the art as particle form polymerizations.

In most commercial scale operations, it is desirable to separate thepolymer and the liquid medium comprising an inert diluent and unreactedmonomers in such a manner that the liquid medium is not exposed tocontamination so that the liquid medium can be recycled to thepolymerization zone with minimal if any purification. A particularlyfavored technique that has been used heretofore is that disclosed in theScoggin et al, U.S. Pat. No. 3,152,872, more particularly the embodimentillustrated in conjunction with FIG. 2 of that patent. In such processesthe reaction diluent, dissolved monomers, and catalyst are circulated ina loop reactor wherein the pressure of the polymerization reaction isabout 100 to 700 psia. The produced solid polymer is also circulated inthe reactor. A slurry of polymer and the liquid medium is collected inone or more settling legs of the slurry loop reactor from which theslurry is periodically discharged to a flash chamber wherein the mixtureis flashed to a low pressure such as about 20 psia. While the flashingresults in substantially complete removal of the liquid medium from thepolymer, it is necessary to recompress the vaporized polymerizationdiluent (i.e., isobutane) in order to condense the recovered diluent toa liquid form suitable for recycling as liquid diluent to thepolymerization zone. The cost of compression equipment and the utilitiesrequired for its operation often amounts to a significant portion of theexpense involved in producing polymer.

Some polymerization processes distill the liquefied diluent prior torecycling to the reactor. The purpose of distillation is removal ofmonomers and light-end contaminants. The distilled liquid diluent isthen passed through a treater bed to remove catalyst poisons and then onto the reactor. The equipment and utilities costs for distillation andtreatment can be a significant portion of the cost of producing thepolymer.

In a commercial scale operation, it is desirable to liquefy the diluentvapors at minimum cost. One such technique used heretofore is disclosedin Hanson and Sherk's U.S. Pat. No. 4,424,341 in which an intermediatepressure flash step removes a significant portion of the diluent at sucha temperature and at such a pressure that this flashed portion ofdiluent may be liquified by heat exchange instead of by a more costlycompression procedure.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to an apparatus for continuouslyseparating polymer solids from a liquid medium comprising an inertdiluent and unreacted monomers. In another aspect, the invention relatesto an apparatus for continuously separating polymer solids from a liquidmedium, drying the polymer, and recovering the diluent and unreactedmonomers with a reduction in compression needed for diluent vaporcondensation to liquid diluent for reuse in a polymerization process. Inanother aspect, the invention relates to a method for continuouslyseparating polymer solids from a liquid medium. In another aspect, theinvention relates to a method for continuously separating polymer solidsfrom a liquid medium, drying the polymer, and recovering the inertdiluent and unreacted monomers for reuse in a polymerization process.

In accordance with the present invention, there is provided an apparatusfor continuously recovering polymer solids from a polymerizationeffluent comprising a slurry of said polymer solids in a liquid mediumcomprising an inert diluent and unreacted monomers. The apparatuscomprises a discharge valve on a slurry reactor, examples of whichinclude slurry loop reactors and stirred tank slurry reactors, for thecontinuous discharge of a portion of the slurry reactor contents into afirst transfer conduit: a first flash tank having a bottom defined bysubstantially straight sides inclined at an angle to the horizontalequal to or greater than the angle of slide of the slurry/polymersolids; wherein the pressure of the first flash tank and temperature ofthe polymerization effluent are such that from about 50% to about 100%of the liquid medium will be vaporized and the inert diluent componentof said vapor is condensable, without compression, by heat exchange witha fluid having a temperature in the range of about 65° F. to about 135°F.: a first flash tank exit seal chamber, communicating with said firstflash tank, of such a length (l) and diameter (d) as to permit such alevel of concentrated polymer solids/slurry to accumulate and form apressure seal in said first flash tank exit seal chamber: a seal chamberexit reducer providing for a continuous discharge of a plug flow ofconcentrated polymer solids/slurry to a second transfer conduit whichcommunicates the concentrated polymer solids/slurry into a second flashtank wherein the pressure of said second flash tank and temperature ofthe concentrated polymer solids/slurry are such that essentially all ofany remaining inert diluent and/or unreacted monomer will be vaporizedand removed overhead for condensation by compression and heat exchangeand the polymer solids are discharged from the bottom of said secondflash tank for additional processing or storage.

The invention provides also a method for the continuous removal of astream of polymerization effluent from a slurry reactor through adischarge valve; increasing the heat content of the polymerizationeffluent during its transit through said first transfer conduit to atemperature below the fusion point of the polymer while continuouslycommunicating the polymerization effluent to a first flash tank having abottom defined by substantially straight sides inclined at an angle tothe horizontal equal to or greater than the angle of slide of theconcentrated polymer solids/slurry; continuously vaporizing from about50% to about 100% of the liquid medium in said first heated flash tankto yield a concentrated polymer solids/slurry and a vapor stream at sucha temperature and pressure that the inert diluent content of said vaporis condensable, without compression, by heat exchange with a fluidhaving a temperature in the range from about 65° F. to about 135° F.;continuously discharging the concentrated polymer solids/slurry fromsaid first flash tank to a first flash tank exit seal chamber of such alength (l) and diameter (d) that a volume of concentrated polymersolids/slurry is continuously maintained so as to form a pressure sealin said first flash tank exit seal chamber; continuously discharging theconcentrated polymer solids/slurry from said first flash tank sealchamber through a seal chamber exit reducer defined by substantiallystraight sides inclined at an angle to that of horizontal equal to orgreater than the angle of slide of the polymer solids which remain afterremoval of about 50 to 100% of the inert diluent therefrom;communicating a continuous plug flow of concentrated polymersolids/slurry from said first flash tank exit seal chamber through saidseal chamber exit reducer to a second transfer conduit whichcommunicates said continuous plug flow of concentrated polymersolids/slurry to a second flash tank; and continuously vaporizingessentially all of any remaining inert diluent and/or unreacted monomerin a second flash tank operated at a lower pressure than said firstflash tank; condensing the vaporized inert diluent and/or unreactedmonomer from said second flash tank by compression and heat exchange;and continuously discharging the essentially dried polymer slurry fromsaid second flash tank for further processing or storage.

An object of the present invention is to provide both an apparatus andmethod for the continuous two stage flash drying of the polymer solidsfollowing the continuous removal of the polymerization effluentcomprising polymer solids and liquid medium comprising inert diluent andunreacted monomers from a slurry reactor through a point dischargevalve, a continuous solids level control in the first flash tank exitseal chamber that provides a pressure seal therein which enables saidfirst flash tank to operate under a substantially greater pressure thansaid second flash tank while polymer solids are continuously dischargedthrough the seal chamber exit reducer into the second transfer conduitand further into the second flash tank which eliminates plugging in thefirst flash tank and the continuous liquification of from about 50% toabout 100% of the inert diluent vapor by heat exchange rather thancompression.

Another object of the invention is to eliminate the need for a settlingleg on the slurry reactor and the intermittent high pressure pulse inthe slurry reactor caused by periodic discharging of the contents of thesettling leg. Another object of the present invention is to improvesafety by eliminating the possibility of plugging in a settling leg.

Another object of the invention is to eliminate plugging in equipmentdownstream from the discharge valve. In a settling leg of apolymerization reactor polymerization continues and the heat of reactionfurther heats the liquid medium and a potential exists for some of thepolymer solids to dissolve or to fuse together. As the contents of thesettling leg exit the discharge valve, the pressure drop causes flashingof some of the liquid medium which results in cooling the remainingliquid medium causing the dissolved polymer to precipitate which tendsto plug downstream equipment. The present invention which eliminates theneed for a settling leg also eliminates this potential for downstreamequipment plugging by avoiding the initial dissolution or fusion of thepolymer solids.

Another object of the present invention is to increase the reactorthrough-put by the use of increased ethylene concentrations in theliquid medium. Settling legs limit ethylene concentrations due to anincreased tendency to plug downstream equipment caused by acceleratedreaction within the settling leg. A continuous polymerization effluentslurry flow allows ethylene concentrations to be limited only by theethylene solubility in the liquid diluent in the reactor, therebyincreasing the specific reaction rate for polymerization and increasingreactor throughput.

Other aspects, objects, and advantages of the present invention will beapparent from the following disclosure and FIGS. 1 and 2.

The claimed apparatus and process provide several advantages over theprior art including: (1) allowing for a continuous processing of thecontents of a slurry reactor from the point of discharge of theliquified polymerization effluent through a discharge valve; a firstflash tank; a seal chamber; a seal chamber exit reducer; and therefromto a second flash tank, (2) significantly increasing ethyleneconcentration in the liquid medium thereby increasing reactorthrough-put and (3) energy consumption is reduced by reducing the needto compress and/or distill the reactor vapor-liquid effluent. Recyclingcompressors and other downstream equipment can be reduced in size oreliminated.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1 and 2 are a schematic diagram illustrating an apparatus forcontinuously separating polymer solids from diluent and unreactedmonomer in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is applicable to any mixture which comprises aslurry of polymer solids and a liquid medium comprising an inert diluentand unreacted monomers including slurries resulting from olefinpolymerization. The olefin monomers generally employed in such reactionsare 1-olefins having from 2 up to 8 carbon atoms per molecule. Typicalexamples include ethylene, propylene, butene, pentene, hexene andoctene.

Typical diluents employed in such olefin polymerizations includesaturated aliphatic hydrocarbons having 3 to 8, preferably 3 to 4 carbonatoms per molecule, such as propane, isobutane, propylene, n-butane,n-pentane, isopentane, n-hexane, isooctane, and the like. Of thesediluents those of 3 to 4 carbon atoms per molecule are preferred, andisobutane is most preferred.

The rate of discharge of the polymerization effluent is such as to allowa continuous process stream from the slurry loop reactor from the pointof discharge of the liquified polymerization effluent through a singlepoint discharge valve and also through the first flash tank and theassociated vapor recovery and solids recovery systems. The rate ofdischarge of the polymerization effluent is such as to maintain aconstant pressure in the slurry reactor and to eliminate intermittenthigh pressure pulses associated with a discharge of a portion of thereactor contents that occurs with settling legs on slurry reactors.

The temperature to which the polymerization effluent slurry which isdischarged from the reactor is heated during transit to the first flashtank for vaporization is below the fusion temperature of the polymer.This may be accomplished by appropriate heating of this first transferconduit. The quantity of heat to be supplied to the polymerizationeffluent during its transit through this first conduit to the firstflash tank should preferably be at least equal to that quantity of heatwhich equals the heat of vaporization of that quantity of inert diluentwhich is to be flash vaporized in the first flash tank. This then willprovide for the concentrated polymer solids formed in the first flashtank to be passed to the second flash tank to pass thereto at a highersolids temperature and thus facilitates the removal of residual diluentin the pores of such polymer solids by the operation of the second flashtank. That quantity of heat transferred to the polymerization effluentduring its transit through the first transfer conduit to the first flashtank may even be greater, provided only that the quantity of heat sotransferred will not cause the polymer solids therein to become heatedto such a temperature at which they will tend to fuse or agglomerate onewith another.

The concentrated polymer solids/slurry are discharged from the firstflash tank into a first flash tank exit seal chamber of such a length(l) and diameter (d) so as to provide a volume sufficient to maintain avolume of concentrated polymer solids/slurry sufficient to maintain apressure seal in the exit seal chamber. The concentrated polymersolids/slurry are discharged from the exit seal chamber through an exitseal chamber reducer to a second transfer conduit which communicates theconcentrated polymer solids/slurry as a plug flow to a second flashtank. The exit seal chamber reducer is defined by substantially straightsides inclined at an angle to that of horizontal equal to or greaterthan the angle of slide of the concentrated polymer solids/slurry.

The pressure for the first flash step will vary depending on the natureof the diluent and unreacted monomers and the temperature of thepolymerization effluent. Typically, pressures in the range of from about140 psia to about 315 psia can be employed; more preferably from about200 psia to about 270 psia; and most preferably from about 225 psia toabout 250 psia.

The heat exchanging fluid used to condense the vapor from the firstflash step is at a temperature in the range of from about 65° F. toabout 135° F. A preferred embodiment uses a heat exchange fluid at atemperature of from about 75° F. to about 125° F. A most preferredembodiment uses a heat exchange fluid at a temperature of from about 85°F. to about 115° F.

A further understanding of the present invention will be provided byreferring to FIG. 1 which illustrates a system comprising an embodimentof the invention.

In the embodiment illustrated in FIG. 1, the polymerization is carriedout in a loop reactor 1. The polymerization mixture is circulated byagitator 2. Diluent comonomer and monomer are introduced from thediluent storage vessel 40, the comonomer storage vessel 41, and themonomer storage vessel 42 through their respective treater beds 37, 38,and 39 through conduits 5, 4 and 3, respectively, connected to conduit6. Catalyst is added through conduit 7. Normally, catalyst is introducedas a suspension in a hydrocarbon diluent.

Polymerization effluent is removed from the loop by continuous dischargethrough the single point discharge valve 8. The polymerization effluentpasses from the discharge valve 8 to a conduit 9 which is provided witha line heater 10 and into the first flash tank 11 which separatesvaporized liquid medium from polymer slurry/solids. Conduit 9 has anindirect heat exchange means such as a flash line heater 10.

Vaporized liquid medium comprising diluent and unreacted monomers exitthe first flash tank 11 via transfer conduit 12 through which it ispassed into a cyclone 13 which separates entrained polymer solids fromthe vapor. Polymer solids separated by the cyclone are passed via line14 through a dual valving assembly designed to maintain a pressure sealbelow cyclone 13 to a lower pressure flash tank 15.

The concentrated polymer solids/slurry in the bottom of the first flashtank 11 continuously settles by sliding along the straight line bottomsurface 16 thereof into the seal chamber 17 which is illustrated inenlargement FIG. 2. A polymer solids/slurry level 43 is maintained inthe seal chamber 17 to eliminate plugging tendencies in flash tank 11and to form a pressure seal so that flash tank 11 can operate at asubstantially higher pressure than flash tank 15. Polymer slurry/solidsare continuously discharged from the seal chamber 17 into the lowerpressure flash tank 15. The length (l), diameter (d), and volume of theseal chamber 17 and the geometry of the seal chamber exit reducer 18 arechosen so as to provide a variable residence time and provide acontinuous plug flow of concentrated polymer solids/slurry to minimize“dead” space and reduce plugging tendencies. The seal chamber 17 lengthmust be sufficient to allow practical level measurement and control.Typical residence times of the concentrated polymer solid/slurry in theseal chamber 17 are from 5 seconds to 10 minutes, preferable residencetimes are from 10 seconds to 2 minutes and most preferable residencetimes from 15–45 seconds. The continuous plug flow of concentratedpolymer solids/slurry forms a pressure seal wherein the concentratedpolymer solids/slurry have an l/d ratio inside the seal chamber 17 whichis typically 1.5 to 8, preferable l/d is 2 to 6 and most preferable is2.2 to 3. Typically the seal chamber exit reducer 18 sides are inclined,relative to the horizontal, 60–85 degrees, preferable 65–80 degrees andmost preferable 68–75 degrees. The seal chamber exit reducer 18 geometryis defined by substantially straight sides inclined at an angle to thatof horizontal equal to or greater than the angle of slide of theconcentrated polymer slurry/solids and communicates the concentratedpolymer solid/slurry to a second transfer conduit 19 which communicateswith a feed inlet of flash tank 15. In flash tank 15 substantially allof any remaining inert diluent and unreacted monomer in the concentratedpolymerization effluent is vaporized and taken overhead via conduit 20to a second cyclone 21.

The major portion of the liquid medium in the polymerization effluentwill have been taken to cyclone 13 as vapor. The vapor after having theentrained polymer solids removed is passed via conduit 22 through a heatexchanger 23 wherein the vapor at a pressure from about 140 psia toabout 315 psia is condensed by indirect heat exchange with a heatexchange fluid such as to eliminate the need for compression.

The condensed liquid medium comprising diluent and unreacted monomers isthen passed to an accumulator 24. A pump 25 is provided for conveyingthe condensed liquid medium back to the polymerization zone by line 26.

The polymer solids in the lower pressure flash tank 15 are passed vialine 27 to a conventional dryer 28. The vapor exiting the secondarycyclone 21, after filtration in filter unit 29, is passed by line 30 toa compressor 31 and the compressed vapors are passed through a conduit32 to a condenser 33 where vapor is condensed and the condensate ispassed through conduit 34 to storage vessel 35. The condensed liquidmedium in the storage vessel 35 is typically vented overhead for removalof light-end contaminants. The inert diluent can be returned to theprocess through a treater bed 37 to remove catalyst poisons or distilledin unit 36 for more complete removal of light-ends and then returned tothe process through a treater bed.

Having broadly described the present invention it is believed that thesame will become even more apparent by reference to the followingexamples. It will be appreciated that the examples are presented solelyfor the purpose of illustration and should not be construed as limitingthe invention.

EXAMPLES Example 1

A typical ethylene polymerization process would be conducted at atemperature of about 215° F. and a pressure of 565 psia. An example ofsuch a process would result in a polymerization effluent of about 83,000pounds per hour comprising about 45,000 pounds per hour of polyethylenepolymer solids and about 38,000 pounds per hour of isobutane andunreacted monomers. The continuously discharged polymerization effluentis flashed in the first flash tank at a pressure of about 240 psia and atemperature of about 180° F. to remove overhead about 35,000 pounds perhour of diluent and unreacted monomer vapors and entrained particulates.Auxiliary heat to impart an additional quantity of heat to thepolymerization effluent is supplied by appropriate heating means duringthe transit between the discharge valve and the first flash tank. Afterremoval of the fines, the isobutane vapor is condensed, withoutcompression, by heat exchange at a pressure of about 240 psia and atemperature of about 135° F. The polymer slurry/solids discharging fromthe bottom of the first flash tank into the seal chamber form acontinuous plug flow of concentrated polymer slurry/solids, whichprovides a pressure seal, with an l/d ratio of the plug of polymerslurry/solids of 2.5 in an 8′4″ long seal chamber having an l/d ratio of5.5 and with a cone angle of about 68° on the seal chamber exit reducer.The residence time of the continuous plug flow of concentrated polymerslurry/solids is about 16 seconds. The concentrated polymerslurry/solids are continuously discharged from the bottom of the firstflash tank at a temperature of about 180° F. and a pressure of about 240psia through a seal chamber, seal chamber exit reducer, and a secondtransfer conduit into a feed inlet on a second flash tank. The remainingliquid medium in the concentrated polymer slurry/solids communicated tothe second flash tank is flashed at a temperature of about 175° F. andat a pressure of about 25 psia to remove about 4,300 pounds per hour ofisobutane and unreacted monomers which are condensed by compression andheat exchange.

Example 2

A typical ethylene polymerization process would be conducted at atemperature of about 215° F. and a pressure of 565 psia. An example ofsuch a process would result in a polymerization effluent of about 83,000pounds per hour comprising about 45,000 pounds per hour of polyethylenepolymer solids and about 38,000 pounds per hour of isobutane andunreacted monomers. The continuously discharged polymerization effluentis flashed in the first flash tank at a pressure of about 240 psia and atemperature of about 175° F. to remove overhead about 23,000 pounds perhour of diluent and unreacted monomer vapors and entrained particulates.After removal of the fines, the isobutane vapor is condensed, withoutcompression, by heat exchange at a pressure of about 240 psia and atemperature of about 112° F. The polymer slurry/solids discharging fromthe bottom of the first flash tank into the seal chamber form acontinuous plug flow of concentrated polymer slurry/solids, whichprovides a pressure seal, with an l/d ratio of the plug of polymerslurry/solids of 2.5 in an 8′4″ long seal chamber with an l/d ratio of5.5 and with a cone angle of about 68° on the seal chamber exit reducer.The residence time of the continuous plug flow of concentrated polymerslurry/solids in the seal chamber is about 16 seconds. About 60,000pounds per hour of concentrated polymer slurry/solids are continuouslydischarged from the bottom of the first flash tank at a temperature ofabout 175° F. and a pressure of about 240 psia through a seal chamber,seal chamber exit reducer and a second transfer conduit into a feedinlet on a second flash tank. The remaining liquid medium in theconcentrated polymer slurry/solids communicated to the second flash tankis flashed at a temperature of about 125° F. and at a pressure of about25 psia to remove about 16,000 pounds per hour of isobutane andunreacted monomer which are condensed by compression and heat exchange.

1. A process comprising: polymerizing at least one monomer in liquiddiluent in a slurry reactor to produce a slurry of polymer solids in aliquid medium; continuously discharging a portion of the slurry from theslurry reactor as polymerization effluent; and flashing thepolymerization effluent in a first flash to vaporize from about 50% toabout 100% of the liquid in the polymerization effluent to produceconcentrated polymer effluent and vaporized liquid; wherein the rate ofdischarge of the polymerization effluent from the slurry reactor is suchas to maintain constant pressure in the slurry reactor.
 2. The processof claim 1, wherein the rate of discharge of the polymerization effluentis such as to eliminate intermittent high pressure pulses in the slurryreactor.
 3. The process of claim 1, wherein the polymerization isconducted in a loop reactor.
 4. The process of claim 1, wherein theconcentrated polymer effluent and vaporized liquid are continuouslyseparated.
 5. The process of claim 1, wherein the concentrated polymereffluent is flashed in a second flash to vaporize liquid.
 6. The processof claim 1, wherein the vaporized liquid from the first flash containsentrained polymer solids and is subjected to a separation to separatepolymer solids from the vaporized liquid.
 7. The process of claim 4,wherein the resulting separated, vaporized liquid from the first flashis condensed by indirect heat exchange.
 8. The process of claim 5,wherein the vaporized liquid from the second flash contains entrainedpolymer solids and is subjected to a separation to separate polymersolids from the vaporized liquid.
 9. The process of claim 5, wherein thevaporized liquid from the second flash is condensed by compression.